The purpose of this Mentored Scientist Research Development Award (K01) is to provide the candidate with mentoring and research experiences that will promote her development as an independent public health researcher with particular methodological expertise in dyadic research methods. Objectives of the training are to develop knowledge and skills in: 1) relationship theory to understand interpersonal dynamics; 2) dyadic study methods including cohort and intervention design; 3) advanced statistical techniques for dyadic data; and 4) ethical conduct of research. Training activities will include didactic courses and specific workshops, directed reading and one-on-one tutorials with mentors, and applied research experience. The candidate will receive mentorship from a Training Committee of internationally renowned experts in the fields of socio-psychological relationship theory, dyadic methods, substance use research, and prevention science for substance using populations. The specific research aims of the proposed K01 are to: 1) measure interpersonal factors and examine their association with high-risk injecting behaviors at the dyad-level; 2) determine the mediating and moderating effects of interpersonal factors on the association between factors exogenous to the dyad and high-risk injecting behaviors; and 3) examine whether changes in levels of interpersonal factors predict high- risk injecting behavior. The research aims will be accomplished by collecting a combination of cross-sectional (n=300 pairs) and longitudinal (n=100 pairs) data from injecting dyads (pairs of individuals who inject drugs together) as part of a multi-site cohort study of HIV and hepatitis C incidence in injection drug users (InC3, PI: Page). The proposed K01 research activities leverage sponsor study infrastructure to pursue questions not posed by the sponsor study by collecting data on interpersonal factors from both members of injecting dyads. Research activities are innovative because they: (1) use a novel measurement scale for interpersonal factors; (2) use dyadic data collected from pairs of individuals who inject together; and (3) will inform dyad-based prevention strategies to reduce intravenous transmission of HIV. Findings from the proposed research will position the candidate to apply for an R34 award to develop, implement, and evaluated a dyad-based risk- reduction intervention targeting interpersonal dynamics within injecting-dyads. The findings generated under the R34 will allow the subsequent submission of an R01 to test the efficacy of a full random control trail for a dyad-based intervention targeting interpersonal factors. In sum, the K01 activities will develop the candidate into a successful independent researcher with a unique methodological skill-set which combines dyadic research methods with her foundation in qualitative and quantitative epidemiological methods.